1. Field of the Invention
The present invention relates to a method for generating a first and a second phase modulated carrier signal from a input signal, in particular a polar modulated input signal, for construction of a driving signal for a power amplifier according to claim 1, and a modulator for generating a first and a second phase modulated carrier signal from a polar modulated input signal according to claim 7.
2. Description of Related Art
Existing types of FM-modulation schemas, for example Gaussian Minimum Shift Keying (GMSK) as in the Global System for Mobile communications (GSM), are using the spectrum relatively inefficiently. One reason for this is the fact that FM modulations do not carry information in the signal envelope. In other words, ideal FM modulations comprise a constant signal envelope, wherein the amplitude of the transmission signal does not carry information. In the next generations of digital cellular radio systems, use of the Quadrature Amplitude Modulation (QAM) will be standard, in order to reach a higher spectrum efficiency.
Polar-modulations, as the afore-mentioned QAM, have a non-constant signal envelope. Accordingly, amplification of such signals requires linear power amplifiers. However, amplifiers in linear mode or linear configuration are less efficient than class-C or class-D power amplifiers currently used, for instance, in FM-type modulators. Further, conventional linear power amplifiers cause heat and/or operation time problems in transmitter equipment, in particular in thermally limited equipment alike mobile terminals.
In order to reduce such effects, new transmitter architectures based on switching mode power amplifiers have been introduced. Switching mode power amplifiers can theoretically reach a power efficiency of 100% and do not alter significantly the phase of the input modulation signal, but they are extremely amplitude non-linear. In this respect, it is known from Linear Amplification with Nonlinear Components (LINC), proposed by D. C. Cox in “Linear Amplification with Nonlinear Components”, IEEE Transactions an Communications, COM-22, pp. 1942 to 1945, December 1974, that any bandpass signal with both amplitude and phase variations can be represented by two pulse modulated signals which are of constant amplitude and have only phase variations. Hence, a proper method may be to convert a complex modulation signal to a respective pulse signal that comprises pulse width modulated (PWM) and pulse phase modulated (PPM) pulses, a so-called phase modulated pulse width modulated signal (PWM-PPM).
Beside the general concept discussed above, there are several ways to control a Power Amplifier (PA) in the prior art. One class of polar-modulation solutions is those where the AM-component is added via modulation of the supply-voltage of the RF Power Amplifier. For instance, U.S. Pat. No. 6,794,931 related to this approach. In order to be power efficient the supply-voltage modulation should be done via an efficient switching DC-DC converter. However, it is difficult to achieve a high modulation bandwidth as well as to get rid of the switching ripple.
A second class of polar-modulation solutions are those where the input of a switching PA is driven with a two-level PWM-signal, which can be generated by comparing the AM- and PM-modulated carrier signal with a triangular (or saw-tooth) signal with a fundamental frequency being at least twice the carrier frequency. Issues with this solution are the high PA switching frequency, causing over-sampling, and the severe linearity requirements on the triangular (or saw-tooth) signal. Besides, the carrier already should have been linearly AM modulated before the PWM conversion takes place.
A third class of polar-modulation solutions are those where the input of the switching PA is driven with a two-level pulse-density signal which may be generated by a bandpass sigma-delta converter as, for instance, described by R. Schier in “Bandpass Sigma-Delta Modulation”, Electronic Letters, Vol. 25, No. 23, November 1989. An important issue with this solution is the required high PA switching frequency, leading to a significant over sampling ratio.